I couldn't find a thread that lists it under the search function, but I apologise if this topic has been covered elsewhere.

As the header says, I've got blue smoke coming from my exhaust from 5000rpm upward. I've read and been told that it could either be rings or seals, but nobody has been able to tell me how to properly diagnose the problem. How can I determine which is the culprit?

I found another 2.5 that has documents proving a bottom end rebuild less than 20,000km ago, however that also blew a small amount of blue smoke of start-up. Does anyone know what the likely cause of this is?

Generally blue smoke on acceleration is from the rings, on decelleration is from the valve stems/seals.

A leak down test can help check the ability of the rings to seal. It is more informative then a compression test. (But a compression test is usually easier to perform.) Adding about 20-30 ml of engine oil to the cylinder and repeating the test can check the rings (the oil temporarily increases the sealing ability of the rings but not the valves).

Another item to check is the brake fluid. Have you been needing to add any? If the brake master cylinder is leaking into the vacuum booster the fluid will get sucked into the intake and then burned. Usually that shows as more of a white smoke but it could be bluish-white.

Thanks, ghnl. I know I had a small fluid leak back into the booster due to a failed master cylnder. I never thought of that. I have since replaced the MC, so I assume that is no longer an issue. I'll get my mechanic to have a proper look.

Any thoughts on blue smoke at startup on a recently rebuilt engine? It sat in a paddock for years after 20000km on the 'new' motor.

The crancase breather there on the valve cover is the problem...to a degree. There isn't a baffle to keep oil from being thrown off the valvetrain into it, the hole in the valve cover is kind of small (though this shouldn't matter a bigger hole would let the vapor escape and let the oil drain back into the crankcase better), and there is a fair bit of vaccum as it is connected downstream of the AFM (would be less if it was plumbed into the airbox between the air filter and AFM).

Run a compression test with the engine at normal operating temperature. You should see about 170psi and no more than 5psi difference between cylinders. If the compression test comes up low on one or all then do a leakdown test. That will tell you weather your problem is valves or pistons/rings.

The above is right...if you have "leaky" rings you have blowby and too much of that will put a bunch of oil in the intake.